Composite materials have a wide variety of commercial and industrial uses, ranging from aircraft and automobile to computer parts. Composite materials have many advantages which make them attractive to different industries. For instance, composite materials can reduce heat transfer, resist conduction of electricity, limit reflection of radar waves, are flexible but strong, and can be fairly easily formed into complex shapes during manufacturing. Some examples of commercial applications include the complex shapes of certain automobiles, airplanes, and boats which would be difficult to form with metal materials. Another important use of composite materials is the creation of stealth aircraft which minimize their radar cross section through the use of radar absorbing composite materials that form the majority of the aircraft's structure.
One example of an aircraft made largely from composite materials is the F-22 Raptor, the world's premier tactical aircraft, designed and manufactured by Lockheed Martin Tactical Aircraft Systems. The Raptor's composite parts are formed with flexible graphite fibers, called a ply, that are impregnated with epoxy or BMI resins which harden when subjected to the application of heat. The uncured plies are placed on tools, each tool corresponding to a composite part of the Raptor. Thus, when the graphite resin mixture hardens over the tool, the composite part is formed with the proper shape.
A number of production techniques are available for forming composite parts. Again, using the Raptor as an example, once the plies are placed over the tool, a vacuum bag is used to hold the plies securely to the tool during curing of the resin. The vacuum bag forces the material to the tool and prevents the formation of bubbles and other material deformities. The tools are then placed in an autoclave for heating. An autoclave is essentially a large oven with the ability to precisely control the thermal energy applied to tools during curing of composite parts. An autoclave operator can monitor and control the amount of thermal energy applied to the tools to maintain a predetermined heating rate of the composite parts. For instance, one typical resin will cure into the strongest possible material if the resin is heated from room temperature to just over 350.degree. F. at a heating rate of between 1.degree. F. per minute and 5.degree. F. per minute. Variations from this heating schedule could result in defective parts.
The autoclaves used for curing of aircraft composite parts are necessarily large in order to accept large parts, and in order to mass produce a large number of parts. For instance, a typical autoclave is 50 feet long and has a heating container large enough to accept a forklift driven into it. The autoclave distributes thermal energy through the heating container by blowing heated air with a large fan located at one end of the heating container.
An autoclave operator must carefully distribute tools in the autoclave heating container in order to ensure that heating rate specifications are met, especially when larger autoclaves are used to produce parts of varied sizes and materials. The operator must ensure that only compatible materials are processed in each autoclave production run. Once the tools are placed in the autoclave, the autoclave operator generally must monitor the temperature and heating rate of the tools to compensate for variances in the thermal energy introduced by the autoclave, interference between tools caused by uneven airflow, and other difficulties. Operator intervention can ensure that the tools are heated within the proper specifications; however, all of this can be time consuming tasks for an autoclave operator. Further, when an autoclave operator attempts to compensate for uneven heating relating to variations in the distribution of thermal energy throughout the autoclave heating container, inefficiencies can be introduced to the production process. For instance, if an autoclave operator adjusts heating rates to a lower level in order to avoid over-heating of a part, the autoclave will require a greater amount of time to cure other parts, increasing the time required for the entire production run. Finally, if the parts are distributed improperly, the autoclave operator may have to violate the heating rate specifications for some of the tools, thus wasting the parts on those tools, in order to obtain useful parts from other tools.